Mitigating catches of endangered sharks species in small-scale fisheries: a case study on pelagic thresher sharks (Alopias pelagicus Nakamura, 1935) in the eastern Indian Ocean

Abstract

Many shark species (Class Chondrichthyes) are threatened with extinction due to overfishing. Of the >1,000 described Chondrichthyan species, the pelagic thresher sharks (Alopias pelagicus) are of particular conservation priority since they are evolutionarily distinct and globally endangered. As the world’s largest shark fishing nation, with particularly high catch rates of thresher sharks, the impact of Indonesian fisheries has become a global priority for thresher shark conservation. Therefore, baseline data is needed to inform management and implement international environmental agreements such as the Convention on International Trade of Endangered Species (CITES). This research provides data and management recommendations for small-scale targeted shark fishing in Western Indonesia, where thresher sharks are regularly caught. Yet, fisheries are data-poor, with limited information on catch rates and limited regulation in place. Daily landings data was recorded from June 2019 to December 2020 in two sites in Southwest Aceh, and analysed to explore catch patterns, risk factors for thresher sharks, and potential mitigation options. The results show that 25 wooden vessels of <30 Gross Tonnage (GT) use bottom longlines to target sharks in the Indian Ocean. A total of 109 pelagic thresher individuals were recorded during the study period, with a female-skewed sex ratio (1 Female vs 0.39 Male), presumably impacting the population's reproductive potential. Based on this data, we offer some potential management measures to reduce fisheries impacts on pelagic thresher sharks by applying a simple mitigation hierarchy framework. These results offer fisheries management recommendations, including effort limits and gear modifications. Overall, the data we present could contribute to understanding the stock status and exploitation rate of thresher sharks in the Indian Ocean and support CITES implementation in Indonesia. At the same time, the mitigation hierarchy approach and recommendations could be applied to other similar small-scale fisheries in Indonesia and globally

Conservation of thresher shark (Family: Alopiidae) in South‐west Aceh

“…Although mostly caught as bycatch, several targeted shark fisheries operating in Aceh, such as in South-west Aceh district, caught more than two thousand individuals of sharks and rays in 2020 (Ichsan et al, 2022) . In this district, we recorded more than 15 shark longline vessels operating in the open ocean and regularly landing in two landing sites of Ujong Serangga and Lhok Pawoh (Figures 1 and 2) (Simeon et al, 2020).…”

“…Most catches comprised of requiem sharks (Family: Carcharhinidae) such as silky shark (Carcharhinus falciformis) and spot-tail shark (Carcharhinus sorrah) (Ichsan, Ula, & Herman, 2021). Catches of thresher sharks are less than 5% of the total elasmobranch landed in South-west Aceh catch (Figure 3); however, threshers catch almost inevitable in every shark-targeted fishing trip (Ichsan et al, 2021(Ichsan et al, , 2022.…”